Semiconductor device and method for producing the same

ABSTRACT

A semiconductor device wherein a coating film which is made of a polyimide resin or a polyimide isoindoloquinazolinedione resin and which is at least 10 μm thick is disposed on at least an active region of a semiconductor substrate, and the resultant semiconductor substrate is encapsulated in a ceramic package. The semiconductor device has troubles relieved conspicuously, the troubles being ascribable to alpha-rays which come flying from impurities contained in the material of the package.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 06/908,782, filed Sept.18, 1986, which is a continuation-in-part of application Ser. No.750,783, filed July 1, 1985, which is a continuation of application Ser.No. 351,665, filed Feb. 24, 1982 (now abandoned), which is acontinuation of application Ser. No. 092,512, filed Nov. 8, 1979, (nowabandoned).

BACKGROUND OF THE INVENTION

(i) Field of the Invention

This invention relates to a semiconductor device of very excellentreliability and a method for producing the same, and more particularlyto a semiconductor device free of any trouble due to alpha-rays from apackage and a method for producing the same.

(ii) Brief Description of the Prior Art

In general, semiconductor devices are sealed usually by the ceramicpackage (including also a method -cerdip- in which the ceramic packageis sealed with glass), the plastic package, or the like. Especially inthe ceramic package among these packages, ceramic materials containuranium, thorium etc. on the order of several ppm. As stated in, forexample, the "16th Proceedings of Reliability Physics (1978)", at page33, it has been known that these impurities emit alpha-rays andtherefore cause failures (called "soft errors") in memory devices etc.For this reason, the reliability of the semiconductor devices may lowerconspicuously.

On the other hand, when powder, such as alumina, called filler is usedin plastic package materials, the impurities are also contained in thefiller. The filler is less influential on the integrated circuit devicesthan in the case of the ceramic encapsulation because it is surroundedwith plastic materials such as epoxy resin and the alpha-rays emittedfrom the impurities are absorbed by the plastics. These plasticmaterials, however, have the disadvantages of comparatively lowreliability in moisture resistance and heat resistance on account of thefact that the moisture absorbability is high, the fact that the contentsof alkali impurities typified by Na are high and the fact that the heatresistance is as low as at most about 150° C. It is therefore difficultto employ plastic-encapsulated integrated circuit devices in systems ofhigh reliability. Accordingly, integrated circuit devices for use in thehigh-reliability systems need to be subjected to the ceramic typeencapsulation.

For these reasons it has been strongly desired to prevent soft error ofthe semiconductor memory device caused by alpha-particles coming fromuranium and thorium contained in the ceramic package as impurities.

In Electronics, June 8, 1978, pages 42-43, it is shown that this problemis attacking from several different directions, for instance, bydevising new packaging material process or applying protective coatingto the upper surface of the chip.

However, this article does not disclose the most favorable materials ornecessary characteristics of the protective coating in order to preventsoft error caused by alpha particles.

IEEE Journal of Solid State Circuits, vol. SC-13, No. 4, August, 1978,pages 462-467 shows planar multilevel interconnection technologyemploying a polyimide resin.

However, in this article, the polyimide films having a thickness of 2.5μm are employed for interlevel dielectrics and final passivation. Itmust be recognized, however, that such thin films of polyimide cannotprevent penetration of alpha particles coming from outside of the filmsuch as the ceramic package.

The use of the polyimide film as the protective film or coating toprevent soft error of the semiconductor memory device caused by alphaparticles is not disclosed in this article.

Also, some prior art references show the use of polyimide resin in thefield of the semiconductor devices, however, none of these referencesshow the use of polyimide as a protective film or coating to preventsoft error of a semiconductor memory device caused by alpha particles.

For instance, U.S. Pat. No. 4,017,886 provides a polyimide layer betweenan SiO₂ layer and a metal layer to bond a wire with the electrode veryeasily by forming a flat upper surface on which the metal layer isformed.

Furthermore, Japanese Patent publications No. 47-12609 and No. 52-26989show the uses of polyimide for insulation and final passivation,respectively.

There is no disclosure in those prior art references concerningprevention of soft error caused by alpha particles.

SUMMARY OF THE INVENTION

This invention has for its object to eliminate the disadvantages of theprior art, and to provide a semiconductor device which maintains a highreliability for moisture and heat exhibited by the ceramic encapsulationwherein the failure of an integrated circuit due to alpha-rays aspreviously stated, is prevented from occurring; as well as a method forproducing such a semiconductor device.

In order to accomplish the object, a semiconductor device according tothis invention has a coating film on at least a region of an element ina semiconductor substrate having at least one element, the coating filmbeing made of a polyimide resin or a polyimide isoindoloquinazolinedioneresin (hereinbelow, written "PII resin") and being 10 μm or more thick,and it is encapsulated in a ceramic package.

The semiconductor device of this invention causes the polyimide resin orthe PII resin to attenuate and absorb alpha particles which emit fromimpurities contained in a package material. Accordingly, the resincoating film to serve as an attenuating material and an absorbingmaterial is required to be a film thick to the extent of preventing thealpha particles from penetrating therethrough. In order to avoid anyfluctuation in the characteristics of the element, the thickness shouldpreferably be at least 10 μm and more preferably be at least 30 μm. Thecapability of preventing the penetration of the alpha particles is notlimited to the resin coating films, but it is generally possessed byinsulating films. It is extremely difficult, however, that insulatingfilms of silicon dioxide, phophosilicate glass, silicon nitride,aluminum oxide etc. having heretofore been employed in semiconductordevices are deposited on semiconductor substrates 10 μm or more. Morespecifically, these insulating films formed by the chemical vapordeposition undergo very high stresses and cause cracks when depositedseveral μm or more. With the sputtering process, the insulating filmscan be deposited under the condition under which the proportion ofdevelopment of the cracks is held comparatively low. However, thedeposition rate is as very low as several hundreds Å/min, and it isactually impossible to deposit the films 10 μm or more. In contrast,with the polyimide resin or the PII resin, the stress of the film is asvery low as about 4 Kg f/mm². In addition, the breaking distortion isabout 30%, which is approximately one order greater as compared withthose of the aforecited inorganic insulating films. Therefore, a thickfilm of several tens μm can be formed under the condition under whichquite no crack develops. On the other hand, among high polymer resins,some possess film forming characteristics similar to those of thepolyimide resin and the PII resin. Since, however, the sealing step ofthe ceramic package is ordinarily executed at high temperatures ofaround 450° C., a heat-resisting property enough to endure thetemperatures is required, and no resin other than the aforecited onessatisfies this property.

More specifically, as exemplified in FIG. 1, when various high polymerresins are subjected to thermogravimetric analyses, decreases in weightbegin at 200°-250° C. in case of a silicone resin 13 and in case of anepoxy resin 14, whereas a decrease starts at 500° C. in case of thepolyimide resin 12. In case of the PII resin 11, the heat resistance ismore excellent, and the weight residue at 600° C. is approximately 70%which is the most excellent. In this manner, the polyimide resin or thePII resin has the heat-resisting property against the high temperaturestep described previously.

In the PII resin or the polyimide resin, the contents of impurities suchas uranium and thorium functioning as alpha-ray generating sources areas very low as 0.1--several ppb or so (the impurity analyses resorted toradioactivation analyses). Accordingly, the PII resin or the polyimideresin stops the alpha-rays emitted from the ceramic package material,and simultaneously, the alpha-rays to be generated by the resin itselfbecome an extremely small amount. On the other hand, it can be generallysaid that organic high polymer materials are lower in the impuritycontents than inorganic materials. However, in case of a polyethyleneresin taken as an example of the organic high polymer material, theuranium content is 40-50 ppb which is comparatively high, and theorganic high polymer materials are not always suitable.

The coating of the polyimide resin or the PII resin attenuates andabsorbs alpha particles coming from uranium and thorium contained in theceramic package, thereby, penetration of alpha particles and soft errorof the semiconductor memory device are effectively prevented.Furthermore, it is necessary that the amounts of alpha particles emittedfrom the coating film itself is extremely low.

The protective coating film to prevent soft error caused by alphaparticles must satisfy following conditions.

(1) The amounts of uranium and thorium contained in the coating filmitself is extremely low, i.e. less than 40 ppb, so that the coating filmdoes not emit alpha rays enough to cause soft error of the semiconductormemory device.

(2) The coating film must have a thickness enough to attenuate andabsorb alpha rays and must never have cracks.

(3) The coating film should be heat resistant so as to withstand heatingin the packaging step (the assembly process includes a step of heatingat 300° to 500° C.).

However, a material which can satisfy these conditions was not knownbefore the present inventors.

The present inventors have found out that only the PII resin and thepolyimide resin can satisfy these conditions from experiments requiringa great deal of expense and labor.

That is, it has been widely believed that organic resin materials surelycontain remarkable amounts of impurities, such as uranium and thorium,because representative refining means, such as recrystallization orzone-refining are never employed to make the organic resin material.

However, the present inventors have carried out studies of the polyimideresin and the PII resin and have made clear by huge amounts of data thatamong many kinds of organic resin material, only the PII resin and thepolyimide resin can satisfy above conditions but other resin materialscan not satisfy them.

It is needless to say that in order to know which organic resin materialcan satisfy above condition (1), it is necessary to know the amounts ofuranium and thorium contained in each resin, respectively.

However, trace analysis of uranium and thorium called for a great dealof expense and labor. The most reliable method of trace analysis ofuranium and thorium is the radioactivation analysis by thermal neutronsin the nuclear reactor and no other methods are available that providedreliable data.

This method involves placing the sample of the organic resin material inthe nuclear reactor, radiating thermal neutrons to activte the sampleand measuring the uranium and thorium contents from the attenuationcurve of the gamma-rays generated upon disintegration of theradioactivated element.

This method requires the use of the nuclear reactor for one full day andattenuation of the gamma-rays must be continuously measured for abouttwo days for the test of one sample. Therefore, to obtain the data offive samples of Table 1 4,000,000 yen and a great deal of effort wasnecessary.

The amounts of uranium and thorium contained in five kinds of organicresin materials measured by the present inventors are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                     CONTENT (ppb)                                                    MATERIAL       URANIUM    THORIUM                                             ______________________________________                                        Polyimide Resin                                                                              0.1˜10                                                                             0.1˜10                                        PII Resin      0.1˜10                                                                             0.1˜10                                        Epoxy Resin    ˜1,000                                                                             ˜1,000                                        Silicon Resin  ˜1,000                                                                             100˜1,000                                     Polyethylene Resin                                                                           40˜50                                                                              ˜50                                           ______________________________________                                    

From Table 1, it is evident that among five kinds of organic resinmaterial, only the PII resin and polyimide resin can satisfy abovecondition (1). However, the other resin materials contain more than 40ppb of uranium and thorium and cannot satisfy condition (1).

Also, it should be recognized that the protective film to prevent thesoft error of the semiconductor memory device must have a thicknesssufficient to prevent the penetration of alpha particles therethroughcoming from the ceramic package.

The capability of preventing the penetration of alpha particles is notlimited to the resin coating film. However, the present inventors foundout from their experiments, that it is extremely difficult to form aninorganic material film having such a thickness, 10 μm or more.

That is, according to the present inventors' experiments, it wasextremely difficult to form protective films having a thickness of 10 μmor more, of silicon dioxide, phosphosilicate glass, silicon nitride,aluminum oxide, etc., on a semiconductor substrate. More specifically,these inorganic insulating films formed by the chemical vapor depositionundergo very high stresses and cause cracks when deposited atthicknesses of several μm or more. With the sputtering process, theinsulating films can be deposited so that development of the cracks issuppressed and the crack development is comparatively low. However, thedeposition rate in this process is very low, i.e. on the order ofseveral hundreds Å/min., and it is actually impossible to form a filmhaving a thickness of 10 μm or more.

Accordingly, it is not possible to satisfy condition (2) using aninorganic insulating material.

In contrast with the polyimide resin or the PII resin of this invention,the stress of the film is very low as about 4 kgf/mm². In addition, thebreaking distortion is about 30%, which is one order greater as comparedwith those of aforesaid inorganic insulating films. Therefore, a thickfilm of more than 10 μm can be easily formed under the condition underwhich no crack develops.

On the other hand, among high molecular organic resins, some possessfilm forming characteristics similar to those of the PII resin and thepolyimide resin. Since, however, the sealing step of the ceramic packageis ordinarily executed at high temperatures of 300°-500° C., aheat-resisting property enough to endure the temperatures is required,and no resin other than the PII resin and the polyimide resin satisfiesthis property.

More specifically, as exemplified in FIG. 1, when various organic resinmaterials are subjected to thermogravimetric analysis, decreases inweight begin at 200°-250° C. in the case of a silicon resin 13 and inthe case of an epoxy resin 14; whereas, a decrease starts at 500° C. inthe case of the polyimide resin 12. In case of the PII resin 11, theheat resistance is more excellent, and the weight residue at 600° C. isapproximately 70% which is the most excellent.

Therefore, it is recognized that among those resin materials, only thePII resin and the polyimide resin can satisfy the aforementionedconditions (2) and (3), the other organic resin materials cannot satisfythe condition (3).

As heretofore described in the PII resin and the polyimide resin, thecontents of uranium and thorium are extremely low. Accordingly, the PIIresin and the polyimide resin stops the alpha-rays emitted from theceramic package, and simultaneously, the alpha-rays to be generated bythe alpha generators in the resin itself are in an extremely smallamount. Therefore, these resins are very superior for the protectivefilm to prevent soft error caused by the alpha rays. In order to avoidany fluctuation in the characteristics of the semiconductor memorydevice, the thickness of the PII resin film or the polyimide resin filmshould preferably be at least 10 μm, more preferably at least 30 μm.Both the PII resin and the polyimide resin can be said to be excellentmaterials also from the standpoint of the impurity contents of uranium,thorium, etc. As previously stated, however, the PII resin is morefavorable due to its heat resistance.

Here, the "polyimide resin" shall mean a polymeric material which isobtained by the reaction between aromatic diamine and aromatictetracarboxylic acid dianhydride, while the "PII resin" shall mean apolymeric material which is obtained by the reaction among aromaticdiamine, aromatic tetracarboxylic acid dianhydride and aromaticdiaminocarboxamide. Both are well known, and the PII resin is describedin detail along with a manufacturing method therefor in, for example,the official gazette of Japanese patent application publication No.48-2956.

Since it is the semiconductor element that is influenced by thealpha-rays, a semiconductor substrate to be used in this inventionincludes at least one element or at least one active region which isaffected by the entrance of the alpha-rays. Since that part of thesemiconductor substrate which is affected by the alpha-rays is a portionof the element region, the coating film of the resin to be disposed forstopping the invasion of the alpha-rays must exist at least on theregion of the element or the active region.

In the presence of an insulating layer, an electrode, an interconnectionlayer etc., the semiconductor substrate shall include them. Thesemiconductor devices of this invention are mainly constructed ofmonolithic integrated circuits.

The ceramic encapsulation is a technique well known in the field ofsemiconductor technology, and all the ceramic packagings having hithertobeen employed can be applied. The ceramic packagings are, for example,those called "cofired ceramic dip" and "cerdip". The ceramics usuallycontain aluminous ceramics as their principal constituents, but any ofthe ceramic materials having hitherto been employed may be used.Further, glass whose principal constituent is lead glass is employed forthe bonding between the ceramics in the cerdip type. In the cofiredceramic dip type, on a ceramic package to which the semiconductorsubstrate is die-bonded, a cover made of a metal such as Kovar or aceramic is bonded by the seam welding or with a binder such as eutecticalloy between Au and Sn.

The polyimide resin and the PII resin (especially the polyimide resin)sometimes contain some (on the order of several ppm) alkali impuritiessuch as Na. In this case, when the semiconductor substrate is formedthereon with the resin film of, for example, the polyimide resin and issubjected to a heat treatment at a high temperature, the alkaliimpurities permeate into the interior of the semiconductor substratethrough pinholes etc. existent in an insulating film disposed on thesurface of a semiconductor sheet constituting the semiconductorsubstrate, and they can change the characteristics of the element. Tothe end of preventing this drawback, it is effective that aphosphosilicate glass film or a silicon nitride film which has a highcapability of checking the permeation of the alkali ions is formed onthe semiconductor substrate and interposed between it and the polyimideresin.

The foregoing semiconductor device of this invention can be readilymanufactured by a producing method including: (i) the step of coveringat least a region of an element in a semiconductor substrate having atleast one element, with a polyimide resin or a PII resin having athickness of 10 μm or more, and (ii) the step of encapsulating in aceramic package the semiconductor substrate covered with the resin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the thermogravimetric curves of various highpolymer resins,

FIG. 2a is a sectional view of a semiconductor substrate formed with ahigh polymer resin film in an embodiment of this invention, while FIG.2b is a sectional view of a semiconductor device in which thesemiconductor substrate shown in FIG. 2a is encapsulated in a ceramicpackage,

FIGS. 3 and 4 are graphs showing the relationships between the thicknessof a PII resin film and the soft error rate due to alpha-rays insemiconductor devices in embodiments of this invention,

FIG. 5a is a sectional view of a semiconductor device in an embodimentof this invention, while FIG. 5b is a plan view of the semiconductordevice shown in FIG. 5a,

FIG. 6 is a sectional view of a semiconductor device in which a resinfilm is formed on a semiconductor substrate by the potting in anembodiment of this invention,

FIG. 7 is a sectional view of a semiconductor device in which, in anembodiment of this invention, a phosphosilicate glass or silicon nitridefilm is disposed on a semiconductor substrate and a resin film is formedthereon,

FIG. 8 is a plan view of a resin film to be compression-bonded onto asemiconductor substrate in an embodiment of this invention, and

FIG. 9a is a plan view of a polyimide resin film tape for use in anembodiment in which the film carrier method is applied to thisinvention, while FIG. 9b is a sectional view of a semiconductor deviceafter the film shown in FIG. 9a has been bonded.

DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Hereafter, the effects of this invention will be described more indetail in connection with examples.

EXAMPLE 1

This will be described with reference to FIG. 2a. A PII resin film 23which is 4-37 μm thick was formed on a 4-kbit NMOS dynamic RAM (RandomAccess Memory) V_(DD) =12 V) 21 whose body was made of a siliconsemiconductor.

The PII resin was applied onto the wafer by the spin-on method, it wassemicured by subjecting it to a heat treatment at 200° C. for 1 hour,and it was provided with the opening of a bonding pad portion 22 byphotoetching with hydrazine hydrate. Thereafter, it was cured bysubjecting it to heat treatments at 350° C. for 1 hour and at 450° C.for 10 minutes. Preferably the heat treatments are carried out innitrogen or an inert gas. As the PII resin, PIQ (registered trademark ofHitachi Kasei Kabushiki-Kaisha in Japan) was employed. Also in variousexamples to be stated below, PIQ was employed as the PII resin. Theresultant substrate was irradiated with alpha-rays of 5 MeV, and thesoft error rate per minute was investigated. The result is indicated inFIG. 3. When the thickness of the PII resin film was 10 μm or more, nosoft error occurred, and it was apparent that the PII resin film has thecapability of stopping the alpha-rays. Further, the element wasencapsulated in a ceramic package (cerdip type) as shown in FIg. 2b, andthe soft error rate was investigated. As a result, the soft error ratewas about 0.08%/1,000 hours when the PII resin film was not formed,whereas it was below 0.003%/1,000 hours when the pII resin film having athickness of at least 10 μm was formed. A similar effect was verifiedwith Pyre-ML (trade name of Dupont Inc. in U.S.A.) being the polyimideresin.

In FIG. 2a, 21' designates an insulating film, 21" an interconnectionconductor layer, and 24 a bonding wire to be connected with an externalconnection lead. In FIG. 2b, 25 designates the external connection lead,and 26 fused glass to hermetically bond ceramic 27 and ceramic 28.

EXAMPLE 2

An experiment similar to that stated in Example 1 was conducted by usinga 16-kbit NMOS dynamic RAM (V_(DD) =5 V). The result is indicated inFIG. 4. As apparent from FIG. 4, the polyimide resin or the PII resinwhose film thickness is at least 28 μm is required in the presentelement. It is accordingly understood that the required film thicknessof the PII resin varies depending upon elements, and it is generallydesirable to make the film thickness 30-40 μm or greater. Among theelements, one formed with the PII resin film 70 μm was encapsulated in aceramic package (cerdip type) and had the soft error rate investigatedin the same manner as in Example 1. As a result, the soft error rate was0.4%/1,000 hours in case where no PII resin was formed, whereas it wasas excellent as 0.002%/1,000 hours in the present element.

It is obvious that the effects exhibited in this example and Example 1are similar when a polyimide resin or PII resin film 53 is disposed ononly an active region 54 as illustrated in FIGS. 5a and 5b, and it hasbeen experimentally verified. In FIGS. 5a and 5b, numeral 51 designatesa semiconductor substrate, numeral 52 a bonding pad portion, and numeral55 a bonding wire. XX' in FIG. 5b designates the section illustrated byFIG. 5a.

EXAMPLE 3

Five kinds of organic polymer resin material shown in Table 1 wereprepared to examine the effect to dynamic RAM soft error which wascaused by alpha particles emitted from the ceramic package and coatedprotective film of the resin material itself.

These resin materials were coated on dynamic RAM chips employing wellknown spin-on technique and practical soft error rate was estimated,where 16 kb, 64 kb dynamic RAM's packaged in cerdip type ceramic packagewere used.

Thickness of coated protective film resin material was 70 μm for allkinds of resin. 1,000-5,000 semiconductor devices for each group weretested for 1,000-10,000 hours. Testing was performed by write-"0" forall semiconductor devices and read-"0" until error "1" appeared forseveral semiconductor devices among total semiconductor devices in eachgroup.

The results were shown in Table 2.

Soft error rate for uncoated semiconductor devices was relatively higherthan resin coated semiconductor devices. These results indicate thatpolymer resin coating mostly prevents transmission of alpha particlescoming from the ceramic package.

                  TABLE 2                                                         ______________________________________                                        Coating        Soft Error Rate (%/1,000 hr)                                   MATERIAL       16 kb DRAM 64 kb DRAM                                          ______________________________________                                        Polyimide Resin                                                                              0.002      0.004                                               PII Resin      0.002      0.006                                               Epoxy Resin    0.05       0.1                                                 Silicon Resin  0.04       0.1                                                 Polyethylene Resin                                                                           0.04       0.07                                                Uncoated       0.4        0.9                                                 ______________________________________                                    

However, remarkable differences of soft error rate among resin coateddynamic RAM groups are observed. It is believed that these differencesof soft error rate are caused by differences of uranium and thoriumamounts contained in resin materials themselves, respectively.

It is recognized, therefore, that the semiconductor memory device havinga protective coating film made of the PII resin or the polyimide resinis most favorable for preventing soft error caused by alpha particles.

Example 4

In this example, a 4-kbit NMOS dynamic RAM was used, a siliconsemiconductor substrate 61 shown in FIG. 6 was die-bonded to the mountof a package, and a bonding wire 62 was bonded by the wire bonding forexternal connection. Thereafter, PII resin 63 was potted and subjectedto heat treatments at 200° C. for 1 hour, at 350° C. for 1 hour and at450° C. for 10 minutes. The thickness of the PII resin film wasapproximately 40-70 μm.

In case where the resin film is formed by the potting as in thisexample, it is desirable that the bonding wire 62 is made at least 30 μmin diameter. This serves to prevent the bonding wire 62 from coming intocontact with a scribe grid 64 due to the shrinkage of the resinattendant upon curing. It is also desirable that an insulating film 61'is extended on the scribe grid 64 in advance. Any failure due to theirradiation with alpha-rays of 5 MeV did not occur in this example,either. The effect in the case where the present element wasencapsulated in a ceramic package (cerdip type) was the same as inExample 1.

Example 5

As stated before, the polyimide resin sometimes contains some (on theorder of several ppm) alkali impurities such as Na. At this time,especially in the case where pinholes exist in an insulating film 71' ora part 76 appears due to the mask misalignment between an openingportion in the insulating film 71' and a metal interconnnection 71" asillustrated in FIG. 7, impurity ions can penetrate through the pinholesor the part to change the characteristics of an element when a polyimideresin film is formed on a semiconductor substrate and subjected to aheat treatment at a high temperature. It is therefore effective that aphosphosilicate glass or silicon nitride film which exhibits a highgathering ability to the alkali ions be formed on the semiconductorsubstrate and interposed between the substrate and the polyimide resin.That is, as shown in FIG. 7, phosphosilicate glass 73 is formed on asilicon semiconductor substrate 71 by the chemical vapor deposition, anda polyimide resin film 74 is formed thereon. The phosphosilicate glassshould preferably have a phosphorus content of 3 mol % to 12 mol % and afilm thickness of approximately 0.3 μm to a 1.5 μm. More specifically,the gathering ability of the phosphosilicate glass film to the alkaliions depends upon the phosphorus content, and an effect demonstratesitself at or above 3 mol %. On the other hand, as the phosphorus contentbecomes higher the moisture absorbability becomes higher, when thecontent exceeds 12 mol % defects such as corrosion of an Alinterconnection begin to appear. The film thickness of thephosphosilicate glass needs to be at least 0.3 μm in order to cover thesemiconductor substrate substantially fully. In addition, the thicknessshould desirably be at most 1.5 μm as a range in which any crackattributed to the tensile stress of the phosphosilicate glass itselfdoes not take place. In this example, a 16-kbit NMOS dynamic RAM wasused, and the phosphosilicate glass 73 at 4 mol % was formed 1.2 μmthick and was formed with the opening of a bonding pad portion 72 by theconventional photoetching. Thereafter, the polyimide resin film 74 wasformed 40 μm thick. This polyimide resin film was formed by a methodsimilar to that in Example 1. In a case where such phosphosilicate glasswas not deposited and where the pinholes in the insulating film 71' andthe mask-misalignment 76 as previously stated existed, the degradationof the available percentage on the order of 0.5-40 % was noted when thepolyimide resin film 74 was formed and subjected to a high-temperatureheat treatment at or above 400° C. In contrast, in this example, thedegradation of the available percentage due to the formation of thepolimide resin film was not incurred, and it was verified that anyfailure due to the irradiation with alpha-rays of 5 MeV did not occur.In the example, the soft error rate in the case of encapsulating theelement in a ceramic package was as excellent as 0.002%/1,000 hours.

Even when the phosphosilicate glass 73 is replaced with a siliconnitride film, a similar effect is confirmed. The silicon nitride filmcan be formed by a known method such as sputtering and plasma (PlasmaEnhanced Chemical Vapor Deposition), and it is desirable to resort tothe plasma CVD. The thickness of the film should desirably lie in arange of from 0.2 to 3 μm. The film thickness of the silicon nitrideneeds to be at least 0.2 μm in order to cover the semiconductorsubstrate substantially completely. The upper limit should desirably setat approximately 3 μm as a range in which the plasma etching of thesilicon nitride film to be stated below can be readily conducted. Thesilicon nitride film can be formed with an opening by, for example, theplasma etching employing CF₄. In FIG. 7, numeral 75 indicates a bondingwire.

It is to be understood that the same effect as above described isrealized even when the PII resin is used instead of the polyimide resinor when the resin is formed by the potting.

Example 6

In this example, a polyimide resin or PII resin film 81 having openings82, as shown in FIG. 8 was compression-bonded onto a siliconsemiconductor wafer. The polyimide resin or PII resin film 81 wascompression-bonded onto the wafer so that the openings 82 correspondedto bonding pad regions of a semiconductor substrate, so that activeregions corresponding to the film 81 were disposed among the bonding padregions, and so that a resin layer was arranged on the active regions.Further, the wafer was put into pellets by dicing, whereupon elementswere assembled. The film was 50-500 μm thick. Here, a 4-kbit dynamicNMOS RAM was used. Any soft error due to their radiation with alpha-raysof 5 MeV did not occur in this example, either. Accordingly, the effectof this invention can be sufficiently expected even when the ceramicencapsulation is employed.

Example 7

A region other than an active region of a silicon wafer formed with anelement was covered with a mesh screen for printing. A prepolymersolution of PIQ (density: 19.5%, viscosity; about 10,000 cp) was appliedby the roller coating from above the mesh screen. Thereafter, thecoating was baked at 200° C. for 60 minutes and further baked at 350° C.for 60 minutes. Thus, a PIQ film being 50 μm thick was formed on onlythe active region. Here, a 4 kbit dynamic NMOS RAM was employed. Anysoft error due to the irradiation with alpha-rays of 5 MeV did not occurin this example, either. Accordingly, the effect of lowering the softerror rate after the ceramic encapsulation can also be sufficientlyexpected.

Example 8

A 4-kbit dynamic NMOS RAM was used, and this pellet was installed on themount of the package. Thereafter, an uncured or semicured film of thepolyimide resin or the PII resin was disposed so as to just cover anelement forming region of the pellet. Thereafter, a the film was bakedat 350° C. for 30 minutes. The uncured film was obtained as statedbelow. A prepolymer solution of polyimide (for example, Torayneece#3000, trade name of Kabushiki-Kaisha Toray in Japan) or a prepolymer ofPIQ was applied onto a flat substrate (for example, glass plate),whereupon it was baked at 100° C. to substantially volatilize off itssolvent. Then, an uncured film which was 30-50 μm thick was formed.Subsequently, the film was cut into a size corresponding to thedimensions of the pellet by means of a sharp cutter and was stripped offfrom the substrate, and the film obtained was put on the pellet of theIC. In the case where by baking was similarly conducted at 200° C., asemicured film which was also 30-50 μm thick was obtained. When theuncured and semicured films were put on the pellets and baked at 350° C.for 30 minutes, they were bonded well with the pellets and performedfunctions as protective films satisfactorily. The semiconductor devicethus produced caused no soft error due to the irradiation withalpha-rays of 5 MeV. In case of the ceramic encapsulation, accordingly,the effect of this invention can be sufficiently expected.

As a modification of the present method, even when instead of the filmpiece of the polyimide resin or the PII resin, an Si piece in the sameshape is bonded onto an active region with a prepolymer of the polyimideresin or the PII resin and then the prepolymer is thermally cured, theeffect is not spoilt. In this case, the Si piece should desirably haveits surface oxidized by the thermal oxidation in advance. Further, thebonding becomes good in such a way that, for example, a pyrolytic filmof an aluminum chelate compound is formed on the surface of the thermaloxidation film as is well known, before the bonding with the prepolymerof the polyimide or PII resin.

Example 9

This indicates an example in the case where a semiconductor device isassembled by the well-known film carrier method. First, description willbe made with reference to FIG. 9b. Here, a 4-kbit dynamic NMOS RAM wasused, and an insulating film 92 was formed on a semiconductor substrate91. For the insulating film there are employed silicon compounds, forexample, silicon oxide and silicon nitride, aluminum oxide etc. whichare formed such methods as sputtering, plasma-enhanced CVD, and CVD. Inthis example, a silicon nitride film 1.5 μm thick can be formed by theplasma-enhanced CVD was employed. A viahole 93 was provided in the filmby the plasma etching employing CF₄, and a bump 94 corresponding to abonding pad portion was formed. In this example, a gold bump being about30 μm thick was formed through an Ni-Cr alloy (500 Å) as well as Pd(2,000 Å) by plating. Subsequently, a tape as shown in FIG. 9a where ina polyimide film 95' located at an active region was formed on advancein a film carrier tape 95 having a thickness of 500 μm was attached tothe semiconductor substrate by thermocompression bonding. Thethermocompression bonding was made between copper foils 96 previouslylaminated on the polyimide film 95 and the bump 94 of gold. Theirradiation with alpha-rays of 5 MeV was thereafter carried out, but theoccurrence of any soft error was not noted. Accordingly, the effect ofpreventing the occurrence of any soft error after the ceramicencapsulation of the resultant semiconductor substrate can also beexpected sufficiently. It is understood that the object of thisinvention is accomplished even when the polyimide film is not in closecontact with the semiconductor substrate as thus far described. Withthis method, after cutting the polyimide film, the semiconductorsubstrate is connected with the package at parts 96' by thethermocompression bonding.

What is claimed is:
 1. A semiconductor device comprising a semiconductorsubstrate having at least one memory element formed therein and a layerfor preventing penetration of alpha particles therethrough provided overat least a region of said memory element, said layer consistingessentially of a resin material selected from the group consisting of apolyimide resin and a polyimide isoindoloquinazolinedione, said layerhaving a thickness of 20 μm or more to sufficiently prevent penetrationof alpha particles therethrough which come from outside of the layer andto avoid soft errors in said memory element, and said resin materialcontaining less than 40 ppb of impurities functioning as alpha raygenerating sources so that the total amount of the impurities will notgenerate alpha rays of sufficient intensity to cause soft errors in saidmemory element.
 2. A semiconductor device according to claim 1, whereinsaid layer has a thickness of more than 28 μm.
 3. A semiconductor deviceaccording to claim 1, wherein said semiconductor substrate isencapsulated in a ceramic package.
 4. A semiconductor device accordingto claim 1, wherein there is no wiring and no bonding pad on said layer.5. A semiconductor device according to claim 1, wherein said layer isprovided on at least an active area of said memory element.
 6. Asemiconductor device according to claim 1, wherein said layer isprovided to cover a major surface and a side wall of said semiconductorsubstrate.
 7. A semiconductor device according to claim 1, wherein atleast one contact hole is provided in said layer.
 8. A semiconductordevice comprising a semiconductor substrate having at least one memoryelement formed therein, a layer having a thickness of 20 μm or moreprovided over at least a region of said memory element, said layerconsisting essentially of a resin material selected from the groupconsisting of a polyimide resin and a polyimide isoindoloquinazolineresin material containing less than 40 ppb of impurities functioning asalpha ray generating sources, soft errors in said memory element causedby alpha rays from said generating sources being prevented by saidlayer.
 9. An integrated semiconductor memory device comprising asemiconductor substrate having a plurality of memory elements formedtherein, a sealing member for sealing the semiconductor substrate and alayer for preventing penetration of alpha particles therethroughinterposed between said semiconductor substrate and said sealing member,said layer having no wiring and no bonding pad thereon, said layerconsisting essentially of a resin material selected from the groupconsisting of a polyimide resin and a polyimideisoindoloquinazolinedione, said layer having a thickness of at least 30μm to sufficiently prevent penetration of alpha particles therethroughwhich come from outside of the layer and to avoid soft errors in saidmemory element, and said resin material containing very lowconcentration of impurities so that the material will not generate alpharays of sufficient intensity to cause soft error in said memory element,and soft error rate of said memory device is not more than 0.002%/1,000hours.
 10. An integrated semiconductor memory device according to claim9, wherein said resin material contains from 0.1 to several ppb of saidimpurities.
 11. An integrated semiconductor memory device according toclaim 9, wherein said layer is provided on an inorganic isulating filmdisposed on said semiconductor substrate.
 12. A semiconductor devicecomprising a semiconductor substrate having at least one memory elementformed therein and a layer for preventing penetration of alpha particlestherethrough provided over at least a region of said memory element,said layer consisting essentially of a resin material selected from thegroup consisting of a polyimide resin and a polyimideisoindoloquinazolinedione, said layer having no wiring and no bondingpad thereon, said layer having a thickness of at least 30 μm tosufficiently prevent penetration of alpha particles therethrough whichcome from outside of the layer and to avoid soft errors in said memoryelement, and said resin material containing from 0.1 to several ppb ofimpurities functioning as alpha rays generating sources so that thetotal amount of the impurities will not generate alpha rays ofsufficient intensity to cause soft errors in said memory element.
 13. Asemiconductor device according to claim 12, wherein said layer isprovided on an inorganic insulating film disposed on said semiconductorsubstrate.
 14. A semiconductor device according to claim 13, wherein awiring is provided between said layer and said inorganic insulatingfilm.
 15. A semiconductor device according to claim 13, wherein abonding pad is provided on said inorganic insulating film.
 16. Asemiconductor device according to claim 13, wherein said inorganicinsulating film comprising a phosphosilicate glass film.
 17. Asemiconductor device according to claim 13, wherein said inorganicinsulating film comprises a silicon nitride film.
 18. A semiconductordevice according to claim 12, wherein the soft error rate of said memoryelement is not more than 0.002%/1,000 hours.
 19. A semiconductor devicecomprising a semiconductor substrate having at least one memory elementformed therein and a layer having a thickness of 20 μm or more providedover at least a region of said memory element, said layer consistingessentially of a resin material selected from the group consisting of apolyimide resin and a polyimide isoindoloquinazolinedione, said layerhaving no wiring and no bonding pad thereon, said layer being interposedbetween said semiconductor substrate and a sealing member for sealingsaid semiconductor substrate, and said resin material containing from0.1 to several ppb of impurities functioning as alpha ray generatingsources so that the total amount of the impurities will not generatealpha rays of sufficient intensity to cause soft errors in said memoryelement.
 20. A semiconductor device according to claim 19, wherein saidlayer is provided on an inorganic insulating film disposed on saidsemiconductor substrate.
 21. A semiconductor device according to claim20, wherein a wiring is provided between said layer and saidsemiconductor substrate.
 22. A semiconductor device according to claim20, wherein a bonding pad is provided on said inorganic insulating film.23. A semiconductor device according to claim 20, wherein said inorganicinsulating film comprises a phosphosilicate glass film.
 24. Asemiconductor device according to claim 20, wherein said inorganicinsulating film comprises a silicon nitride film.
 25. A semiconductordevice according to claim 19, wherein said layer has a thickness of atleast 30 μm.